Nanofluids: a new generation of high efficiency fluids

Nanofluids are a new class of fluids which promise to significantly enhance thermal, rheological and tribological properties of technological fluids. They are obtained by dispersing solid nanoparticles (diameter <100 nm) made by metallic oxides, metals, carbon nanotubes ecc. (Figure 1) in common fluids such as water, glycol, oils and refrigerants. E.g., also at relatively low concentrations of nanoparticles, it is possible to get strong enhancements of thermal conductivity and heat exchange coefficients, with a corresponding enhancement of energy efficiency of plants and components employing such fluids. Other parameters influencing the performance of nanofluids are material, shape and size of nanoparticles, Zeta potential and pH of the colloidal solution, type and concentration of dispersants. Many are the potential applications for nanofluids in HVAC&R. As secondary fluids, they can be applied in commercial refrigeration, chiller, solar panels in absorption systems. As nanolubricants, they can improve thermal dissipation, anti-wear and extreme pressure properties of compressors lubricants. The dispersion of nanoparticles directly in the refrigerant can improve the thermodynamic performance of refrigerating machines. In this paper, the state of the art for the main thermal and tribological properties of nanofluids will be presented with special reference to HVAC&R applications.